In coal mining, it has been common practice to apply limestone in the form of a dust to the walls of a mine, thereby causing the limestone to adhere to the walls. The process, known as “rock dusting,” has two effects. First, because the limestone dust covers exposed surfaces of unmined coal, it prevents mine fires from being propagated along those exposed surfaces. Second, if methane, coal dust, or a mixture of methane and coal dust, ignite in a mine causing an explosion, the rock dust adhering to the mine wall will become airborne, and suppress the propagation of fire resulting from the explosion.
The United States Mine Safety and Health Administration has established standards for rock dusting, which include a requirement that all exposed surfaces of a mine be covered with rock dust at least 80%; of the content of which is non-combustible. Existing methods for applying rock dust include application of rock dust to a mine wall. Recently, mines have begun using chemical foam to achieve improved adhesion of the rock dust to mine surfaces. One method of using foam in rock dust application is to apply a dry mixture of rock dust and a foaming agent to a mine wall. Another method is to apply a mixture of foam and rock dust to a mine wall. In the last-mentioned method, the foam is formed, mixed with rock dust in a mixing vessel, and pumped through a conduit to the point of application. A system for utilizing foam to enhance the adhesion of rock dust to a mine wall is described in U.S. Pat. No. 6,726,849, granted Apr. 27, 2004.